We have demonstrated QT interval prolongation to moxifloxacin in a pilot study of 20 subjects recruited at Massachusetts General Hospital. Apparently healthy male and female volunteers between 18 and 50 years old were eligible if they: were free of known cardiovascular, renal, hepatic disease, had no personal or family history of sudden cardiac death, used no prescribed or over-the-counter medications, had no bradycardia or QTc prolongation on electrocardiography, and had a normal potassium and magnesium. Subjects received 400 mg of oral moxifloxacin or placebo on different days at exactly 9AM in the morning (diurnal variation in QT interval is well recognized17) in the fasting state (to avoid interference with absorption) and had six ten-second electrocardiograms after a minimum of ten minutes of rest, recorded every half hour for six hours. Mass spectrometric analysis of moxifloxacin levels in plasma at 2 and 6 hours after administration closely matched prior reports18 with tight ranges consistent with its known high bioavailability (2hr: 2.8 (SD 0.47) and 6hr 2.64 (SD 0.48) ng/μL). We observed a 12.3 msec increase in heart rate-adjusted QT interval averaged over hours 2-6 compared to baseline after moxifloxacin exposure compared to placebo exposure (ΔΔQTc = +12.3 msec, SD 8.3; Noseworthy, manuscript in preparation).